Voltage boosters are often integrated in memory devices and other integrated devices to provide higher voltages than the supply voltage. These higher voltages may be necessary in nonvolatile memory devices for erasing and writing data or for enhancing the driving conditions (overdriving) of output power devices or for other purposes. A commonly used architecture is shown in FIG. 1, the control phases of which are shown in the diagrams of FIG. 2.
Referring to the figures, the operation of the circuit is as follows. During a first phase of a digital signal (timing signal), the switches formed by the transistors M1 and M3 are closed, while the switch represented by the transistor M2 is open. In this way the capacitor C, biased through the control phase FX at the common reference potential (ground) of the circuit charges to the supply voltage Vdd through the transistor M1, while the load capacity CL discharges to ground through the transistor M3 driven to a state of conduction by the control phase FN. The control phase FBN is at the supply voltage by keeping the transistor M1 turned on while the control phase FBX is at the common reference potential (null voltage or ground) by keeping the transistor M2 turned off.
During the successive phase, the transistors M1 and M3 are turned off by the switching of the respective control phases and the capacitor C, biased at the supply voltage by the switching of the control phase FX, charges the load capacity CL through the switch M2 that in this phase is turned on because of the switching of the control phase FBX. As it may be easily recognized, an output voltage is produced on the connection node of the load capacitance CL that substantially tracks the signal FX, that is a null voltage during a first part or phase of a cycle of the timing signal and a voltage equal to VHT that satisfies the inequality Vdd<VHT<2Vdd, during the second part or phase of the cycle.
Such a circuit as shown in FIG. 1 driven by the phases FBX and FBN of amplitudes equal to the supply voltage Vdd has a number of drawbacks: for relatively low supply voltages, as it is often the case in battery powered devices, the dimensions of the transistors become a crucial parameter and must be relatively large; during the transfer of electric charge from C to CL, a voltage equivalent to a turn on threshold voltage of the transistor switch is lost and the output voltage VOUT is limited to the value 2Vdd−Vth; and being the circuit used to boost the level of a certain digital signal, it is necessary to realize an appropriate circuit for generating the control phases as a function of the input digital signal to be boosted.